Co-sintered surface metallization for pin-join, wire-bond and chip attach

ABSTRACT

A co-sintered ceramic substrate structure is formed through punching and screening of a plurality of ceramic green sheets with a plurality of composite metal pastes and/or inks and laminating the structure. The co-sintered surface metallization comprises dual screened composite metal pastes, one on top of the other and bonded to the sintered ceramic substrate to provide bonding of the surface metallization to the sintered ceramic substrate.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional of application Ser. No. 08/360,526filed Dec. 21, 1994 now allowed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to the field of pin joining,wire bonding and chip attachment to ceramic structures, and theirmanufacture, and more particularly to multilayer ceramic substrates formounting semiconductor devices. In such applications, co-sintering thesurface metallization to facilitate direct pin joining, wire bonding andchip attachment hold significant advantages.

2. Description of the Prior Art

The current art for surface metallization of glass ceramic is done bylithography and thin film evaporation and sputtering on planarizedsurfaces. The need for a planar surface for lithography and thin filmprocesses warrants surface machining of sintered ceramic, and suchplanarization processes are very expensive manufacturing operations.

For substrates having alumina as the ceramic, the metallization isusually molybdenum (Mo) or tungsten (W). The surface metal features arebonded to the ceramic by the addition of glasses. Subsequent joining ofpins, wires and chips to these refractory metal features is done afterplating of nickel on Mo-glass or W-glass composite.

In the case of low temperature sinterable glass ceramics, the metallicconductors used are traditionally copper, silver or gold. The bonding ofsurface metal features to glass ceramic substrates is more difficult dueto large thermal expansion mismatch between the substrate and the metal.One approach for surface metallization of glass ceramic is to depositthin film features either by evaporation or sputtering. This requiresplanar surfaces and hence involves surface machining which makes it veryexpensive. Various class ceramic compositions are disclosed in U.S. Pat.No. 4,413,061 to Kumar et al. Sintering cycles using hydrogen and watervapor have been taught in U.S. Pat. No. 4,234,367 to Herron et al. Thisis done at low temperatures to remove binders.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provideco-sintered surface metallization on ceramic substrate surfaces andmethods of manufacturing the same for use in electronic applications.

It is another object of the present invention to provide surfacemetallization on ceramic substrates without expensive lapping andpolishing.

It is further an object of the present invention to provide a method forachieving direct pin-attach, wire-bond and chip-attach, such thatsurface finishing of the sintered ceramic and/or lithographic or thinfilm metallization is eliminated.

According to the invention, a co-sintered ceramic substrate structure isformed through punching and screening of a plurality of ceramic greensheets with a plurality of composite metal pastes and/or inks andlaminating the structure. A glass ceramic substrate comprising a metalpaste of 40% by volume of copper and 60% by volume of glass as disclosedin U.S. Pat. No. 5,073,180 to Farooq et at., the disclosure of which isincorporated by reference herein, is preferably used in a first layer. Asecond layer including nickel is applied over the first layer. Theco-sintered surface metallization comprises dual screened compositemetal pastes, one on top of the other and bonded to the sintered ceramicsubstrate. Accordingly, it is yet another object of the presentinvention to provide bonding of the surface metallization to thesintered ceramic substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, aspects and advantages will be betterunderstood from the following detailed description of a preferredembodiment of the invention with reference to the drawings, in which:

FIG. 1 is a cross-sectional view showing a stack of ceramic green sheetswith top ceramic layer screened first with composite metal paste forsurface metallization following by a second surface metal paste; and

FIG. 2 is a cross-sectional view showing a sintered ceramic substratewith the co-sintered surface metallization and pin attached metalstructure.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

Referring now to the drawings, and more particularly to FIG. 1, there isshown a plurality of ceramic green sheets 1, which are punched formingvias and screened with metal pastes for electronic applications forminga capture pad. A copper/glass composite paste, such as has beendisclosed in U.S. Pat. No. 5,073,180 to Farooq et al., has been screenedonto the green sheets 1 partially tilling the punch hole 3 and forming afirst layer 2. A second layer 4 of metal paste is deposited on top ofthe first layer 2. Table 1 below outlines 3 combinations of which thefirst and second layers may consist. In Table 1, the preferredcomposition of the first layer is 40% by volume of copper and 60% byvolume of glass of the glass/copper composite with copper and glassparticle sizes taught in the patent to Farooq et al.

                  TABLE 1                                                         ______________________________________                                                                          bonding                                     combination                                                                            first layer                                                                              second layer  atmosphere                                  ______________________________________                                        1        screened   screened nickel                                                                             99% steam                                            glass/copper                                                                             paste         1% hydrogen                                          composite                                                            2        screened   screened (vol. %)                                                                           99.75% steam                                         glass/copper                                                                             copper (88%) and                                                                            0.25%                                                composite  nickel (12%) paste                                                                          hydrogen                                    3        screened   screened (vol. %)                                                                           99.5% steam                                          glass/copper                                                                             copper (80%) and                                                                            0.5%                                                 composite  nickel (20%) paste                                                                          hydrogen                                    ______________________________________                                    

The pastes which comprise the first and second layers are composed ofsubstances which may have different coefficients of expansion from theceramic green sheet. The first layer is formulated such that it remainsbonded (in contact) with the ceramic after sintering and no gaps areformed between the ceramic and the first layer. The first layer, becauseof its composite nature, is closer in coefficient of expansion to thatof the glass-ceramic. The second layer is likewise formed with respectto the first layer, so that there may be a gradation of coefficients ofexpansion between the ceramic green sheet and the second layer, but thethree materials remain in one structure after sintering. (For example,the coefficient of expansion of glass-ceramic is 3 ppm/°C. and that ofglass-ceramic/copper composite is about 7 ppm/°C. The coefficient ofexpansion of the second layer is in the 13 to 18 ppm/°C. range.)

After the first and second layer pastes are applied, the structure thusfar is sintered. Generally speaking, the sintering procedure is doneaccording to the teachings of Farooq et al., U.S. Pat. No. 5,073,180,with a slight change necessary to accommodate the bonding of the firstand second layers to the glass ceramic body. Briefly, the structureundergoes pyrolysis, binder burnoff, and densification according toFarooq et al. Then, the atmosphere is changed to one of the atmospheres:as shown in Table 1 to accomplish sealing and recrystallization of theglass ceramic substrate and bonding of the surface metallization. Thisbonding atmosphere is somewhat different from the sealing atmospherepreferred in Farooq et al. Thereafter, the structure is cooled.

The bonding atmosphere is specifically chosen to provide an environmentin which residual oxide remains to promote bonding. Changes made in thebonding atmosphere could result in no bonding occurring.

FIG. 2 shows the invention with the co-sintered surface metallizationand pin attached. As can be seen in the figure, there is an additionalvery thin layer of plated gold 5 having thickness of about 1000 Å. Thepin head 7 is attached with solder 6, which may be 95% tin and 5%antimony or lead-tin solder.

The invention was tested with a standard pin-pull test. Table 2 showsthe results of the pin-pull test.

                  TABLE 2                                                         ______________________________________                                        Structure type pin-pull, lbs                                                                           Total pins                                           ______________________________________                                        1              11.2 ± 0.9                                                                            90                                                  2              10.7 ± 1.0                                                                           104                                                  3              12.0 ± 0.8                                                                            70                                                  ______________________________________                                    

It is best if failure does not occur in the ceramic. With the presentinvention, failure is expected at four points:

1. At the interface between the ceramic and the copper glass composite9.

2. At the interface between the copper/glass composite and thecopper/nickel or nickel film 10.

3. In the glass ceramic 1.

4. In the pin shank 8.

It is most desirable that failure occur in the pin shank 8. If failureoccurs in the pin shank or solder, the solder can be cleaned out and annew pin added. If the gold layer is pulled off, it can be added againfollowed by replacing the pin and solder. Failure at other interfaceswill require the structure to be built again. Failure in this glassceramic 1 cannot be repaired, thereby necessitating the disposal of theglass-ceramic body. Failure of the structure according to this inventionwas most often observed at the interface between the copper/glasscomposite and nickel or copper/nickel film 10. Failure in the pin shankoccurs at about 12.5 pounds of force. However, the strengths as shown byTable 2 above were high. No failure was observed in the interfacebetween the ceramic and the copper glass composite 9 or the glassceramic 1.

The invention is formed by providing ceramic green sheets, stacked andpunched. Screening a paste of glass ceramic/copper composite onto thestacked green sheets. Screening a paste of nickel or copper/nickel alloyon the composite. Sintering the structure so that the composite has atotal thickness of 6-20 microns and the nickel layer has a totalthickness of 6-20 microns. The structure is plated with gold aftersintering. The structure is then ready for pin joining, wire bonding,and chip attach. The steps of producing this structure do not requireplanarization.

While the invention has been described in terms of preferredembodiments, those skilled in the art will recognize that the inventioncan be practiced with modification within the spirit and scope of theappended claims.

Having thus described our invention, what we claim as new and desire to secure by Letters Patent is as follows:
 1. A ceramic electronic package comprising:a glass-ceramic substrate having at least one via; and a capture pad on said substrate to contact said via, said capture pad comprising a first composite layer of 50-70 volume percent glass-ceramic material and 30-50 volume percent copper in direct contact with said via followed by a second layer comprising nickel or a copper-nickel alloy.
 2. The ceramic electronic package recited in claim 1 wherein the coefficient of expansion of the glass-ceramic substrate is 3 ppm/°C., the coefficient of expansion of the first composite layer of glass-ceramic/copper is about 7 ppm/°C. and the coefficient of expansion of the second layer containing nickel is in the range of 13 to 18 ppm/°C.
 3. The ceramic electronic package recited in claim 1 wherein said second layer is nickel.
 4. The ceramic electronic package recited in claim 1 wherein said second layer is a copper-nickel alloy.
 5. The ceramic electronic package recited in claim 4 wherein said copper-nickel alloy is 88% by volume of copper and 12% by volume nickel.
 6. The ceramic electronic package recited in claim 1 wherein said copper-nickel alloy is 80% by volume of copper and 20% by volume of nickel. 